1. Field of the Invention
This invention relates to a liquid crystal composition for twist nematic (hereinafter abbreviated to TN) mode and a liquid crystal display element of TN mode using the above-mentioned composition. More particularly it relates to a liquid crystal composition capable of reducing the threshold voltage (hereinafter abbreviated to Vth) of TN mode liquid crystal display elements and improving the temperature dependency thereof.
2. Description of the Related Art
In recent years, the performance of liquid crystal display devices have been improved and their fields of application have been being broadened. When introduced liquid crystal display devices were mainly used for handy type electronic calculators, watches, clocks, etc., making use of the specific feature of the small power consumption thereof. Recently, however, the fields of application of the devices, in combination with development of liquid crystal compositions, have come to be rapidly broadened toward those of large-size devices such as outdoor display devices, instrument panels for automobiles, and further, display devices for hand-held computers, terminals of computers, etc.
From the viewpoint of the driving mode of display devices relative to the foregoing, since the quantity of information has increased and the number of parts has been reduced, the 3 V drive static mode at the initial period has come to be transferred to 3 V drive, 1/2 duty in the case of watches, etc., 3 V drive, 1/3 to 1/4 duty in the case of electronic calculators, and 4.5 V drive, 1/16 duty and ten and several V drive, 1/64 duty or further, highly functional display having higher driving voltage and multiplex number in the case of graphic displays such as those of word processors, computer terminals, etc. Further, in addition to these movements, need for low voltage drive as one more specific feature of liquid crystal display devices, has come to be intensified in order to realize making LCD smaller and more lightweight. For example, 1/2 to 1/4 duty in the case of watches, electronic calculators, etc. has been desired to be driven at 1.5 V, 1/16 duty in the case of word processors, etc., at 3 V, and 1/64 duty in the case of computer terminals, etc., at 10 V or lower. Namely, it has been desired to reduce the threshold voltage in the voltage-brightness characteristic of liquid crystal compositions, but it has been very difficult in the case of conventional materials to effect low voltage drive, while satisfying specific features for securing a display quality as mentioned later, i.e. a sufficiently low optical anisotropy value for securing a broad viewing angle and a small temperature-dependency value of threshold voltage.
Usually, in the case of a multiplex drive mode employing a voltage-leveling method, there is an operational restriction which does not occur at all in principle in the case of a static mode. Namely, cross-talk is liable to occur in a half-selected element or unselected element. Furthermore, the higher the multiplex, the less the allowance of the operating voltage, in short, the operating margin; hence cross-talk is more liable to occur.
The operating margin depends on the following three factors: .circle.1 temperature dependency of threshold voltage, .circle.2 viewing angle dependency of threshold voltage and .circle.3 steepness of threshold voltage.
As to the factor .circle.2 among these three, a product of the thickness of display cell (hereinafter abbreviated to d) by the optical anisotropy value (hereinafter abbreviated to .DELTA.n) i.e. a light path length of birefringence (hereinafter abbreviated to d.multidot..DELTA.n) has a great influence upon the factor .circle.2 . For example, it has generally been carried out to obtain a display element of broad viewing angle, by satisfying the following conditional equation (1) derived from Gooch-Tarry's equation (J. Phys. D.: Appl. Phys., vol. 8, 1975) ##EQU1## wherein T: light transmittance, .theta.: twist angle, u: .pi.d.multidot..DELTA.n/.theta..lambda., and .lambda.: wavelength: EQU d.multidot..DELTA.n=550nm . . . (1)
In general, it is difficult in the aspects of cell preparation and product yield to make the cell thickness (d) 5 .mu.m or less; hence the .DELTA.n value of liquid crystal materials is preferred to be less than 0.11 in view of the above equation (1). The above factor .circle.3 is specific of TN mode liquid crystal display devices, and no notable difference in the factor .circle.3 occurs depending on materials. The above factor .circle.1 has a relation also with the operating temperature range, and is most affected by materials. In general, as a countermeasure to liquid crystal compositions in order to improve the above factor .circle.1 , the proportion of a material having a negative dielectric anisotropy value (hereinafter abbreviated to .DELTA..epsilon.) has been increased to thereby reduce the temperature dependency of threshold voltage. However, an increase in the proportion of a material having a negative .DELTA..epsilon. raises the threshold voltage; hence a sufficiently low voltage drive could not have been achieved.
Further, in the case of display devices, the response rate is a nonnegligible and important factor. It is well known that the response rate of liquid crystal display devices has a correlationship with the viscosity of liquid crystal compositions relative thereto.